This invention relates to liquid-level gauging.
The invention is more particularly concerned with ultrasonic liquid-level gauging sensors.
Ultrasonic liquid-level sensors utilize the fact that ultrasonic vibrations travel freely in a liquid but are rapidly attenuated in air or other gas. If an ultrasonic transducer is mounted on the base of a liquid reservoir so that it directs energy up towards the liquid/air interface, the energy will be reflected back down to the transducer by this interface. By measuring the time taken between transmission and reception of an energy pulse, it is possible to measure the distance between the transducer and the liquid/air interface and, from this, the depth of liquid.
It is common practice for ultrasonic transducers of this kind to be mounted at the lower end of a tube that extends from the bottom to the top of the liquid reservoir. The tube is open at the bottom so that liquid fills the tube to the same depth as in the reservoir outside the tube. The tube serves several purposes. It helps isolate the transducer from other sensors or sources of interference. It also confines the ultrasonic beam, so that it is directed only at the region of the liquid surface directly above the transducer. Furthermore, the tube produces within it a region of liquid surface that is substantially damped of waves.
Another advantage arising out of the use of the tube is that it is easy to provide a reference height, by mounting some form of reflector at a known height within the tube. In this way, the transducer will receive a reflection from the liquid surface and one from the reference reflector against which the liquid height can be calibrated. This enables the ultrasonic gauging system to compensate for different liquids having different acoustic propagation properties and for temperature variations which can affect ultra-sound propagation. An example of an ultrasonic probe having a tube of this kind is described in, for example, EP 0106677.
There are various problems with existing ultrasonic liquid gauging sensors. One problem arises from the fact that, in addition to the ultra-sound energy being transmitted through the liquid within the tube, energy is also propagated within the wall of the tube itself. This can lead to propagation from the wall into the liquid, especially at locations where the tube wall is clamped for support purposes, and hence to false echoes being received by the transducer. Attempts to reduce this problem have included the use of tubes made from plastics materials which are less prone to the generation of stray echoes. This, however, causes another problem in that, because plastics are not as rigid as metals, the wall of the tube has to be relatively thick in order to produce the necessary rigidity with a consequent increase in weight. In aircraft fuel gauging applications where a dozen or more fuel gauging probes might be used, the weight can be considerable and lead to appreciable increases in operational costs.
A further difficulty with these sensors is caused where ultrasonic energy does not pass axially along the tube since this gives rise to multiple echoes, and an increase in path length with a consequent time delay. This is especially a problem where the liquid surface is not at right angles to the tube axis since a majority of the reflected signals will not pass axially along the tube. Although the signals that are reflected axially along the tube can be sufficient to enable the height of liquid to be determined, these signals can be masked by strong signals caused by multiple reflection from the wall of the tube.